In order to use a polyolefin resin for a molded product, the polyolefin resin should have superior toughness, strength, environmental stress, crack resistance and melt strength etc. Such characteristics of polyolefin using Ziegler-Natta and metallocene catalysts can be easily improved by increasing molecular weight of the polyolefin resin (polymer), however, this disadvantageously deteriorates the moldability of the polyolefin resin. In order to compensate for the disadvantages of such a polyolefin resin, polyolefin resins having different physical properties are used in combination, but there is a disadvantage that defective appearance of the molded product may occur. Due to such disadvantage of polyolefin resin, it is preferable to use a polyolefin resin having uniform physical property while adjusting the structure of the polymer or using a suitable processing aid, rather than using a mixture of polyolefin resins each having different physical properties. However, generally the polyolefin resin prepared with Ziegler-Natta and metallocene catalyst has narrow molecular weight distribution Thus, if used alone, various problems may occur. When the polymer having broad molecular weight distribution or multimodal molecular weight distribution is used, the moldability of the polyolefin resin can be improved with maintaining characteristics of toughness, strength, environmental stress, crack resistance and melt strength etc., thereby solving the disadvantage of the polyolefin resin having narrow molecular weight distribution.
The polyolefin having multimodal molecular weight distribution is a polyolefin containing at least two components each having different molecular weight, and for example, includes a high molecular weight component and a low molecular weight component in relatively proper proportions. Many studies have been conducted for the preparation of a polyolefin having broad molecular weight distribution or multimodal molecular weight distribution. One method among them is a post-reactor process or a melting blending process in which polyolefin having at least two different molecular weights are blended before or during the processing of the polyolefin. For example, U.S. Pat. No. 4,461,873 discloses a blending method of physically bending two different kinds of polymers for preparing a bimodal polymer blend. When such a physical blending method is used, it is liable to produce a molded form having high gel component, a product appearance is deteriorated owing to the gel component, and thus the polyolefin cannot be used for the films. Further, the physical blending method requires a complete uniformity, so there is a disadvantage of the preparing cost being increased.
Another method for preparing polyolefin having multimodal molecular weight distribution, for example bimodal molecular weight distribution is to use a multistage reactor which includes two or more reactors. In the multistage reactor, a first polymer component having one molecular weight distribution among two different molecular weight distribution of the bimodal polymer, is prepared in a certain condition at a first reactor, the first polymer component prepared is transferred to a second reactor, and then a second polymer component having different molecular weight distribution from that of the first polymer component, is prepared in a different condition from that of the first reactor, at the second reactor. The above-mentioned method solves the problems relating to the gel component, but it uses the multistage reactor, so the production efficiency may be decreased or the production cost may be increased. Also, when the high molecular weight components are prepared in the first reactor, the low molecular weight components are not prepared in the second reactor and thus the finally manufactured polyolefin particles may be made only by the high molecular weight components.
Still another method for preparing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution is to polymerize the polyolefin by using a mixture of catalysts in a single reactor. Recently, in the pertinent art, the various attempts have been made for producing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution, by using two or more different catalysts in a single reactor. In this method, the resin particles are uniformly mixed in a level of sub-particles, thus the resin components each having different molecular weight distribution exists in a single phase. For example, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose a method for preparing polyethylene having bimodal or multimodal molecular weight distribution by using a metallocene-type catalyst comprising at least two metal compounds and the usage of the polyethylene for manufacturing films, pipes, hollow molded articles and so on. Polyethylene produced in this way has a good processability, but the dispersed state of the polyethylene component in the molecular weight per unit particle is not uniform, so there are disadvantages of rough appearance and unstable physical properties even in relatively good processing conditions.
U.S. Pat. No. 4,937,299 discloses a method for preparing polyolefin by using a catalyst system comprising at least two kinds of metallocenes each having different reactivity ratio with respect to monomer to be polymerized. U.S. Pat. No. 4,808,561 discloses a method for preparing olefin polymerization supported catalyst by reacting metallocene with alumoxane in the presence of a carrier. The metallocene is supported in the carrier to form solid power catalyst. As the carrier, inorganic oxide materials such as silica, alumina, silica-alumina, magnesia, titania, zirconia and the mixture thereof, and resinous materials such as polyolefin (for example, finely divided polyethylene) can be employed, and the metallocenes and alumoxanes are deposited on the dehydrated carrier material.
Generally, since the polymer prepared with Ziegler-Natta catalyst alone or metallocene catalyst system has a narrow molecular weight distribution, it is not made to prepare the satisfactory polyolefin which has a multimodal molecular weight distribution or broad molecular weight distribution. Accordingly, in the related art, a method has been known for preparing a bimodal resin by using a mixture catalyst system containing Ziegler-Natta catalyst and metallocene catalyst components. The mixture catalyst system typically includes a combination of heterogeneous Ziegler-Natta catalysts and homogeneous metallocene catalyst. The mixture catalyst system is used for preparing the polyolefin having a broad molecular weight distribution or bimodal molecular weight distribution, to adjust the molecular weight distribution and polydispersity of the polyolefin.
U.S. Pat. No. 5,539,076 discloses a mixture catalyst system of metallocene/non-metallocene for preparing a specific bimodal high-density copolymer. The catalyst system is supported by an inorganic carrier. The carrier such as silica, alumina, magnesium-chloride and the mixture catalyst of Ziegler-Natta and metallocene are disclosed in U.S. Pat. No. 5,183,867, European publication No. 0676418A1, European Patent No. 0717755B1, U.S. Pat. No. 5,747,405, European Patent No. 0705848A2, U.S. Pat. No. 4,659,685, U.S. Pat. No. 5,395,810, European patent No. 0747402A1, U.S. Pat. No. 5,266,544 and WO 9613532A1 etc. The mixture catalyst of Ziegler-Natta and metallocene supported has relatively low activity than single uniform catalyst, so it is difficult to prepare polyolefin having properties suitable for a specific use. In addition, since polyolefin is prepared in a single reactor, the gel which is generated in the blending method may be produced, it is difficult to insert comonomer to high molecular weight components part, the form of polymer produced may be poor and further two polymer components may not be uniformly mixed, so the quality control of the produced polyolefin may be difficult.
Journal of Rheology, 57 (2), 393-406 (2013) discloses that when a polyethylene resin having a multimodal molecular weight distribution is extruded, the polymer chains and the low molecular weight chains are separated from each other due to the difference in elastic energy between the bimodal molecular weights, so that each component therein is not uniformly mixed. This can lead to problems of reducing physical properties and surface defects such as melt fracture, gel formation, etc.